The refining of pig iron generally involves the removal of some of its carbon content by reaction with the supplied oxygen which may be blown in from above and/or bubbled up from the bottom of the melt. Since this oxidation is an exothermic process, solid materials such as scrap iron and iron-rich ores are also introduced to control the bath temperature. The amount of scrap that can be added depends on the proportions of carbon and other oxidizable elements such as silicon, phosphorus and manganese present in the melt; this amount can be increased by igniting the combustible gases--especially carbon monoxide--evolving from its surface.
In commonly owned Luxembourg Pat. No. 81,207 (see also copending U.S. application Ser. No. 222,716 filed Jan. 5, 1981 by Paul Metz et al, now U.S. Pat. No. 4,334,922) there has been described such an afterburning process according to which oxygen blown in from above is spread over the bath surface to react with the CO. A concurrent injection of substantially inert gas from below controls the thickness of the slag layer floating on the bath and prevents it from assuming a foamy consistency which in earlier processes causes that layer to act as a heat insulator. As a result, heat from the afterburning process penetrating the slag layer raises the surface temperature of the bath and enables the utilization of larger quantities of scrap iron and other cooling materials.
A lance supplying oxygen for both decarburization and afterburning must therefore be designed to direct a primary jet through the slag layer into the melt and to train additional jets onto the bath surface. A known nozzle head provided for this purpose has a main axial tuyere surrounded by several ancillary tuyeres whose axes lie skew to the central nozzle axis. These prior nozzle structures, however, do not always operate satisfactorily and in some instances have been found to ignite the evolving CO in a region relatively remote from the bath surface, thereby reducing the heat available for scrap melting and even endangering the lance itself as well as the exhaust system overlying the ladle.